Tasco made a one-piece base for Weaver-style rings. The same base was sold by Tasco with different model/part numbers, sometimes just the base and sometimes the base and rings as a set. They regularly show up on eBay. I bought a couple sets of them within the the last six months. Tasco referred to the one-piece bases as "bridge mounts" and some of the bases use 'BM' as a prefix for the model number.
The last time that I looked, GPC was selling a new swing-over rings and base combo for the Savage 99 for around $25.
