Oh, oh! I was too tired this morning. Instead of "et in rure fuit ... " "hoc in rure ..." sounds far better.
So let's try it again:

Macdonaldo seni rus;
hi-a-hi-ha-ho.
Hoc in rure fuit bos;
hi-a-hi-ha-ho.
Cum muh-muh hic,
muh-muh ibi;
hic muh, ibi muh,
ubique muh-muh.
Macdonaldo seni rus;
hi-a-hi-ha-ho.

...
hoc in rure fuit sus ...
cum oink oink ibi ...

...
hoc in rure fuit mus ...
cum quic quic ibi ...
and so on.

Cantibus gaudete
Wilfried