i've used regular- lt wt muslin in the centers before- when i needed just a little more 'body' than the top/backing provided- it does add a little weight- but not as much as batting- make sure you wash the cotton for the batting and the flannel- they will not shrink/behave the same when finished- they need to be pre=washed....but other than that there is no reason you can't do it if you want- i like the ones i've used muslin in.