This paper presents the XUMA open FPGA overlay architecture. It is a free (as in both free speech and free beer), open-source, cross-compatible embedded FPGA architecture that is intended to overlay on top of an existing FPGA, in essence an "FPGA-on-an-FPGA." XUMA is an island style architecture that can be compiled into a host commercial FPGA. This approach has a number of benefits, including compatibility of the bitstream between different vendors and parts, compatibility with open FPGA tool flows, and the ability to embed some programmable logic into systems on FPGAs without the need for releasing or recompiling the master netlist. These options can enhance design possibilities and improve designer productivity. Previous attempts to map an FPGA architecture into a commercial FPGA have had an area penalty of 100x at best. Through careful architectural and implementation choices to exploit low-level elements of the host architecture, XUMA reduces this penalty to 40x. Using the VTR (VPR6) academic tool flow, we have been able to compile the entire MCNC benchmark suite to XUMA. We invite authors of other tool flows to target XUMA as well.