By double bass I'm assuming you mean like an upright acoustic bass. If so here's a pretty simple one I made. It doesn't have a radiused fingerboard so it's not for bowing - just plunking.
It has decent volume and can easily be miked at the sound port if needed. It's tuned E,A,D,G as normal and uses weedwhacker line. I think a set of actual strings would help the tone but they're pretty pricey.
This design came from Dennis Havlena's fine plans.
Thanks for the advice, she wants to use a bow so I will bee to radius the fretboard.
Question 2. I have made a basic upright bass befor and used standard bass guitar strings, as these are wound would they damage the bow?
What strings did you use?
I used hardware store weedwhacker strings which have a pretty good tone when plucked but don't really bow very well - too slidy. I'm not sure how regular bass guitar strings would affect the bow hairs but I have used a bow on guitar strings with no ill effects. Good quality strings are pretty pricey but a compromise might be a set like this
Which I haven't tried but have heard good things about.
if you are going for a 41.3in scale length (common orchestral double bass size) you will need orchestral bass strings, electric bass guitar strings aren't long enough.
If you are looking at building a 34in scale length (fretless electric bass guitar turned upright sized) then go with flatwound strings so that she can bow it as well as pluck.
Thanks for the advice. Orchstralstrigs are expensive so it look like I will go with a shorter scale .
Very good advise here.