[Duboisi]

You could cover the bottom with wax-paper or cling-film(possibly with paper underneath for visibility), and then brush some vaseline on it.

Or, you may get some sturdy plastic that you cut into a suitable size.

I have found some 3mm white plastic-sheet that I think was 150*50cm in a construction store(for roofing under terraces). They might not have the same sheets at your side of the pond though.

 

[11x]

try tack paper turned upside down and held down with tacks

 

[Stone]

Sorry but I don't know what "tack paper" is. Where can I get it?

 

[11x]

you can get it at dolar genaral or family dollar or lowes. it is often called shelf paper or contact paper. it is the stuff people use to line thier kitchen cabinet shelfs with it is usely thin with some sort of decoritive patern on one side and sticky on the other.here is a link to lowes. make shure to get the adhesive kind. i usaly find it at dollar general for a few bucks for 10-30 feet.
http://www.lowes.com/pl_Shelf+Liner...ewSearch=true&ddkey=http:SearchCatalogDisplay

 

[Stone]

Sure! I remember that stuff! I remember when I was a kid, people were plastering it all over the place - tables, counters, kitchen cabinets! People went wild for it. Thanks!

 

[11x]

i never tried it but it is on my to do list.

 

[Zonker]

There is spray on sticky available from drafting stores, but I had the impression that it doesn't need to be very sticky to catch the mites. I've heard PAM mentioned.