Hi Balthus,
As the scale is 1/76 and not 1/72 there are fewer manufacturers and options available to you, however here are a few ideas for you:
Mount a 37mm flak (SdKfz 7/2) -
http://www.milicast.com/shop/product.php?produc... Mount a Quad 20mm flak (SdKfz 7/1)-
http://www.milicast.com/shop/product.php?produc... The whole ground carriage was mounted on the flatbed of the half-track and you will have to make mesh sides, not as difficult as it sounds, but is time consuming. I have found this link which shows a 1/35 model of the 7/2 which you could use as a guide. Just type SdKfz 7 into Google Images and browse for some interesting shots.
All the best and happy modelling,
Paul