well sure you can spend a lot on curtains if you’d like but you can also get cheap(ish) ones that do the job lol i have blackout Ikea curtains in all my bedrooms. they look great and only cost about €60 a pair.
ah right that makes sense
I always travel with eye patches for this reason!! I sleep better when its dark. I’ve been on pet sits in the USA with no curtains or shades/blinds on a massively busy street too!
We usually pinch cardboard from the supermarket and create makeshift blackout blinds.
A childhood brought up on Blue Peter has made me a dab hand with some scrap card, an empty toilet roll holder, an egg box, a washing up bottle, four wire coat hangers, and some sticky-backed plastic.
@Colin Are you brewing your own hooch as well? 
Use the yoghurt cartons as tumblers.
If the owners have curtains in their rooms but none in the guest room, that is very inconsiderate of them.