There were slimline forearms made specifically for the HK93/33 line in the 70's and 80's but production has since stopped. From time to time you will find a few floating around on GB or the PRO site but it isn't often and they are pricy due to their rarity.
Here is a link to a modded G3 style:
http://www.ar15.com/mobile/topic.html?b=6&f=9&t=335783 you could give it a try if you are a tinkerer. (If the link above doesn't show just do a google search for HK93 slimline and you will find the link among others)
FYI, there is a wood one made by RDTS on the HKPRO site that is $70 if you are interested in that.