Adding mass to Peerless passive radiator

I snared one of the few remaining Peerless XLS 10" passive radiators. (I built an extra box when I built my one sub several years. Wouldn't want to waste that box!)

Problem is, the box is sized for a 400 gram PR, and the only one I could find was the 265 gram version. Has anyone added weight to one of those? I suppose there is a bolt hole under the paper label on the back. What kind of washers or other weights are good for the job? Does anyone have info, or know where I could find info, on how much M5 bolts and various washers weigh?