Are there any other positions (not sitting) one can try while meditating? It is hard for me to sit straight for too long due to my sciatica.

For some meditations I sit but for most I lay flat. I use a small bean pillow to support my neck but my head is in line with the surface I am laying on. I do this straight flat position so I don't fall asleep. When I am tired I lay on my side and listen to a meditation until I peacefully fall asleep…. Go Well

you can try a slow walking meditation if you want to do something dynamic. or you can lie down and put your hands either by your sides or on your heart. I use this lying position when I'm trying to feel into my deepest vulnerabilities.

I very often float in warm water in my bath – lights off
The weight dynamics change in water and muscle relax in warm (salted even!) water
And I can let go entirely – including my neck area muscles tensions

I mean surely any position you want. Prioritise your back and being comfortable over the "right" way to sit. So, however you're most comfortable I guess – lying, sitting back with a back support, standing on one leg (??). The important bit is the mental exercises, not the posture.

I use a meditation bench (https://www.meditationbench.com) instead of a cushion to help with back pain. It’s been a game changer for me.

I’d also recommend walking meditation and some yoga that’s more meditative.

Either lying down flat or you could try walking or movement meditation some even do standing meditation or in asana positions of yoga.

A zen Buddhist once said, there are 4 positions to meditate in 1 is sitting. 2 is standing. 3 is laying down and 4 is walking.

Pain is, by far, the worst deterrent in any activity. It is therefore essential that either 1) you can ‘ignore’ or ‘overcome’ your pain and continue; or 2) you find a way to minimise your pain. But to answer this question directly, I would say without hesitation of a doubt you can meditate in any position you want.
For me, meditation doesn’t always need to take 10 or more minutes each and every time. So, during my ‘short meditation’ moments, I actually do it while walking, standing in line at a grocery store – at any moment in time where I feel that I can just focus on one thing: the sensation of feeling air enter into my body (I feel it best in my nostrils) and then as the air leaves (again, nostrils). I think to my self two words: on my in breath, I think ‘One’ and on the out breath I think ‘Thing’; therefore effectively thinking ‘One Thing’. Because of these 2 words plus the focusing on the sensation of inhaling and exhaling, I can do shorter, more focused breathing at any time of the day, for any period of time and in any position. I’ve even practised this in the middle of a tennis match when I needed to calm down and really focus.
So, yes… you can choose your own position so long as you can focus on your breathing.

There are some variations which you can lay down straight with a cushion against you hear. They are usually for 15 minutes or longer meditations

I do my morning meditation in bed, but be careful that you don't fall asleep haha! I guess a slouch works best for me, though I don't think that's how you're supposed to do it .

Yes, you can practice meditation using yoga Nidra.
You can perform this act by lying down. Pls search for app in App Store.

Lying down on your bed while meditating is just as effective and easier on the body than sitting. Just make sure your head is propped up on a couple pillows and wherever else you need them.

Sit with your back supported against something, I do. It is not so important that you sit with a such a straight posture that you cause yourself pain. No matter how you sit, which I feel you should, just make sure you’re upright and making a conscious effort to sit up a little straighter and you don’t have to over do it. I’d highly suggest inquiring with Sam Harris with anymore questions and highly recommend his app, Waking Up. I’ve mediated for a number of years now and no other app or way of mediating has compared to his.

My back can't handle upright sitting for too long due to past injuries, so laying down is my best option for me. It is what works best for me, though laying down does make it difficult not to fall asleep sometimes!

I normally meditate right before going to bed. I normally lay down flat on my back in my bed. However I know some people find this difficulte as they just fall asleep. You could try sitting but leaning with your back against something to take the pressure out of your back. Or with crossed legs sitting on the floor leaning against the wall for example. Just give different positions a try and figure out what is best for you

Try different yoga positions
Childs Pose for example
Relax the stretch and deep breathe
Meditation is about clearing the mind, not breaking the back
Everyone is different, so find a position to suit you, doesn't necessarily need to be the same each time

I have fibromyalgia, so I understand this struggle. Propping up my hips so that they are elevated higher than my knees helps me sometimes. I also change the position of my legs frequently to alleviate pain.

When I can't sit down at all, I sometimes use a recliner or lay down with my hands resting on my abdomen.