DonW and non-material failure specialists-
According to my 3rd edition Mechanics of Materials text, acrilic and a few other polymers should be drilling at least 1.25 times the thickness of the material. The drilled hole should appear 1/4th the thickness of the material passed the last visible sign of the crack, which the surface should be ground to look for (on something clear like acrylic, I wouldn't bother though). It should then be plugged with a compressive interference fit with a material that features the same thermal expansion properties (aka, use an acrilic rod). The surface compression gained from this method is a contributing factor towards spreading out future loading.
For the folks wondering why, its because the crack damage actaully extends quite a ways passed the area where you last visibly see the crack. A 1/32" hole would be slightly better than not drilling at all, but you would require a crystal ball to know where to locate it in a position to do any good.
For this application being such light load, I recomended a simple 1/4" stop-drilling with an acrilic dowel plug because it doesnt need to be super strong, it just needs to stop spreading. After looking things up, I should have recomended larger, but I still think 1/4" will do fine.
If you wana learn about drill stopping, here are a couple of links that involve stopping cracks in aluminum skin. Due to the modulas and structure of acrylic, it actually needs larger holes than aluminum, because the invisible damage transmits further up from the tip of the visible crack, but I will rest it.
Real non-technical one with examples in aluminum
http://www.fatiguetech.com/coldEX_stop_crack.html
Here is a slightly more technical one.
http://www.dsto.defence.gov.au/publications/1937/
I'm sure there are lots of better data and links online, but I'm not feeling an obligation to search them out and post them.
In reguards to BigT's advise about repairing the body of the crack, I would definately listen to that guy. He is highly experienced, and an artist with acrylic.