Something for you to think about, you could go with a rectangular tube instead of a solid block to reduce weight. Something like this might work:
http://www.industrialmetalsales.com/1972.html
It's 12.75mm tall instead of 9mm, and 25.4mm wide instead of 20mm. I think 6063 is just a more corrosion-resistant version of 6061, and it should definitely be strong enough (the walls are 1/8" [3.17mm] thick).
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz