Someone used to sell acrylic plates for the LR (the ones with the 120 logos on them). I have a piece of 1/2" I’ve been meaning to use for that purpose. I think it would be fine, but I don’t know what thickness you’d need. If you can’t afford a replacement, then stick with something we know works.
I agree that acrylic can be a pain, I’ve snapped enough bits building mine, but I don’t think you will find MDF less prone to breaking.
From what I’ve been able to find it looks like acrylic takes almost 2,000 times more load to break than MDF while still being as stiff. The parts I broke most often were the corner blocks; MDF’s higher co-efficient of friction should result in less force being required to hold the rods, but I doubt it’s enough less to reduce the danger of over tightening.
Acrylic is also easier to put back together after breaking a part than MDF.
Ply is a much stronger alternative with a closer max load, although it is over twice as stiff. The biggest problem with it is that 5 mm Ply doesn’t necessarily mean the board will be 5 mm thick it could be 4.5->5 mm. The only parts, I can remember, where this is significant are the X carriage and the X motor mount plates. So to use ply these would need to be altered to take into account whatever size the ply actually is.
The numbers I found are:
Acrylic 3.2 GPa young’s modulus 72.3 MPa modulus of rupture
MDF 3.8 GPa young’s modulus 32.8 KPa modulus of rupture
PLY (marine ply of unspecified wood) 7.5 GPa young’s modulus 4 Mpa modulus of rupture
The difference between the modulus of Rupture of MDF and Acrylic seems very high so if anyone could check them that would be appreciated.