The most basic Makefile would perhaps look something like (note, this is a home-made Makefile):
SRCS = forcedeth.c
OBJS = $(SRCS:.c=.o)
obj-m += $(OBJS)
EXTRA_CFLAGS = -O2
$(MAKE) -C /lib/modules/`uname -r`/build M=$(PWD) modules
$(MAKE) -C /lib/modules/`uname -r`/build M=$(PWD) clean
$(RM) Module.markers modules.order
However, since I do not know anything about 'forcedeth.c' nor any dependencies it may have with other modules, I doubt this Makefile will work, but nevertheless, you may want to give it a try.
Btw, I am surprised that your kernel does not offer the forcedeth driver, since it appears that many modern Linux distros offer it with their kernels. Is it possible that your kernel has the source for 'forcedeth.c', but that it is not being built because of your kernel configuration? It would verify this first before proceeding down another path.