LR57 - As stated in an earlier post - selecting the right material is key - vinyl is definitely best for this purpose
But I think I know where you are going with this :-)
Take a look at my DIY Vinyl Crown Moulding - last component entry on my "System"
https://systems.audiogon.com/systems/3760
I have not tried this, but I think it might work very well - especially with 1/8" MLV
If MLV is too heavy you could used vinyl upholstery material - the ends may be an issue, but...
- you could cut a Styrofoam form at each end to glue it to and then caulk to finish
Let me know how it turns out.
Regards... :-)))