So I only looked this up since you’ve mentioned it, I hadn’t heard of this before but it definitely can be toxic to hamsters if chewed.
This was a study done on rats:
Repeat oral dose toxicity studies of melamine in rats and monkeys - PubMed
“ Oral administration of melamine at 700 mg/kg/day for 14 consecutive days in rats produced compound-related clinical signs (red urine), decreased body weights, and changes in clinical pathology (increased serum urea nitrogen and creatinine) and anatomical pathology (renal tubular cell debris, crystal deposition, and hyperactive regeneration of renal tubular epithelium). ”
That being said this study is a direct high dosage of its pure form. If what you have made is flat surfaced and unchewable then being around it does not have the same effect, it’s only if ingested. I have read online even some cages do use it but unbranded ones or unverified etc
If it’s not being chewed it up to your discretion but if the risk is there and there is availability to make in a safer material I’d recommend it.
Also I know some waterproof papers are biodegradable and can be made naturally but as it’s easier to chew you may indirectly cause other digestive issues so it may be a double edged sword.