Posting comments as answer…
It depends entirely on the structure of the alternative - if the bowl heats up before the contents, then you are dealing with shielding & most 'simple' calculations go right out of the window. Instead invest in higher temperature plastics… These are given away free with supermarket instant microwaveable meals & if looked after will last for decades.
The things they sell microwave meals in are to all intents & purposes 'microwave invisible'. They don't slow the process at all. They're made of HPET[1] if you want to do some health research, but practically I've had some of these things maybe 20 years. They don't really soften with heat & even hot fat doesn't scar them[2].
I don't buy much microwave food, so I don't pick them up often & then reuse them until I manage to eventually break them. They're great for a can of beans, or leftovers re-heated for tomorrow's lunch etc, or as plate covers if someone gets in late & has to mike their dinner…
& rather irritatingly, even crockery from the same set isn't the same. We have soup-, side- & dinner-plates and cereal bowls from the same set, all ostensibly 'porcelain'. All the plates are microwave invisible, the cereal bowls get red hot without the contents heating much at all.
[1]I once did a fair bit of research on this, when I was trying to find replacements for old, cracked plastic-ware without having to buy the bloody awful meals they contain, but my research was a dead-end. I had to eat the damn stuff to get the dishes, or buy them by the pallet in 10,000s.
Here's a beginner's guide to plastic types, by 'embossed symbol'. My newer ones class themselves as 'PET PP 05' [& no, I don't know what that really means.]
Hunker - What Are the Numbers That Say That Plastic Is Safe to Use in the Microwave Oven?
[2]Conversely, some of my expensive 'tupperware' [not by brand, just by type of 'permanent long term freezer to microwave or keep food in the fridge] type dishes & bowls are permanently scarred & discoloured if there is any kind of oil or fat on the food being microwaved.