It has been said that taking B5 thickens hair strands so perhaps you could try that. Also, I'm not sure what you mean about hair being too thin for twists. What happens when you twist? Do you mean your hair just doesn't look full? Coz I have thin hair strands so I never got the fullness of people with a lot of hair that has a wider diameter. But I found that after doing the twists on dry hair and then wetting them (whether it is doing a CW or just dunking in ACV solution) made them shrink and look fatter, not to mention that letting them drip-dry then completely airdrying gave me twists that hang straight down.
Because I didn't use a leave in, CW were a regular practice. I should've been doing DCs as well, but somehow forgot about them. So when my hair grows back to a length that I am happy wearing in twists (I had a BC in Feb/March), then I will be sure to incorporate weekly DCs into my regimen.
In the pic below, the twists at the top of my head in front show how full and thick my twists look when I wet them and allow them to drip dry. They do shrink but would shrink a lot more if I had applied products as I'll demonstrate shortly. The twists at the back were redone after my hair dried, and as you can see, they appear a lot more sparse because my hair strands are very fine.
Now here's the effect of products on my hair - particularly creamy products:
Compare this twist-out done on hair w/o any products - the top two pics below (I allowed my twists to dry after a wash then undid the twists and voila!)
...with how the same twist-out looked when products were applied:
Go figure.
Shrinkage does make the hair appear fuller so if products do this to your hair, maybe twist then spray sth. If you don't want to wet your twists - especially after applying products, perhaps a baggy might get them to puff up and appear fuller.