what is good boundary to use to prevent antenna power radiation outside a box?

I would like all radiated power from antenna to be in an enclosure (box) to maximize power absorbed by an object. What boundary shall I use? lossless?

Thanks

I am a newbie... Tried designing RMSA in HFSS. I used radiation boundary option available in it. Hope it helps you out.

The radiation box has to have dimensions of lamda/4 from each cordinate extremity.

Friis transmission formula is wat you need