If you can safely get electricity to your hutch then one of these Petnap whelping pads might be useful.
Flexiguard Pet Heat Pad | Petnap Metal Heat Pad chewproof
I have one for my girls in their hutch in a shed. It doesn't get hot but it keeps at just above body temperature, they all snuggle up together on it when it's cold.
I put mine under a couple of sheets of newspaper & a thin layer of very small shredded paper.
I put an old duvet down over the hutch at night too.
I used to use the Snugglesafe's they are very good. You can get for around £13 when they're on offer (make sure you get the actual Snugglesafe, as often shaped spare covers can be mistakenly bought for a similar price - the Snugglesafe usually comes with a basic fleece cover anyway).
I'm amazed you can manage without a microwave - I'd be lost without mine now

Most of my vegetables I do in it, no need to add salt like I would in the pan.
Could u add a little shelf for the microwave if space on your worktop is limited?
Boxes stuffed with hay are good for keeping Guineas warm.
Or if it's an outdoor hutch, maybe at night/very cold or windy weather put a piece of clear perspex/plywood/cardboard put up to the mesh (leave an inch or two gap for ventilation & to stop condensation at the top), would cut down a lot of the draughts (this is generally more of a problem than the temperature).
An old duvet put down over (make sure ventilation) with a waterproof cover will all help to insulate the hutch & keep them warmer.
Good luck xx